When the LTE standard began, the requirement on improvement of cell edge data rate was proposed. To fulfill such a requirement, a variety of solutions have been put forward in the industry, including the well-known soft frequency reuse and fractional frequency reuse, among others. Currently, these technologies are being widely studied and applied, and have been developed into a key LTE feature as inter cell interference coordination (ICIC).
The frequency band used in fractional frequency reuse is divided into two portions, each portion using a different reuse factor. As shown in FIG. 1, the B1 (=B11+B12+B13) portion uses reuse factor 3, while the B2+B3 portion uses reuse factor 1. The curve in FIG. 1 illustrates the upper power density limit (UPDL) of a transmitter on each portion. The power density of the transmitter can be smaller than or equal to such UPDL, which, however, cannot be exceeded. This curve is also termed power density mask (PDM for short).
The B1 portion with reuse factor of 3 uses the frequency band with a relatively high UPDL, which can cover the entire cell; while the B2+B3 portion with reuse factor 1 use a relatively low UPDL, which can cover an inner region of the cell only. The coverage areas of fractional frequency reuse are illustrated in FIGS. 2 and 3, of which, FIG. 2 shows the scenario of omnidirectional cells, and FIG. 3 shows the scenario of three-sector cells. The two scenarios are similar to each other. For the sake of convenience, the omnidirectional cell and the three-sector cell are both termed cell in the present invention.
The symbols of frequency band in FIGS. 2 and 3, i.e., B11, B12, B13, B2, and B3 are placed at the inner sides of the farthest boundaries of their corresponding coverage areas. For example, B2 and B3 are placed at inner positions of the cells, which indicates coverage of only the inner cell region; while B11, B12, and B13 are placed at the edges of the cells, indicating coverage of entire cells, including, of course, inner regions of the cells, although not marked in the figures.
FIGS. 2 and 3 show regular hexagonal cells, wherein the inner region of a cell is the area inside a regular circle or ellipse. In practice, due to the complex radio environments, a cell is usually in an irregular shape, with a complex boundary between the inner region and the cell edge.
The cell edge users are subjected to relatively severe interferences. Reuse factor 3 can be used to prevent interferences from adjacent cells, thereby improving signal-to-interference-plus-noise ratio (SINR). Interferences at the cell center are relatively slight, and thus reuse factor 1 can be used to make full use of the bandwidth. Therefore, fractional frequency reuse has attracted much attention.
The fractional frequency reuse can be applied in both uplink (signals travel from a terminal to a base station) and downlink (signals travel from a base station to a terminal).
The major defects of fractional frequency reuse lie in that a cell cannot use the whole bandwidth, and even fewer bandwidth is available at the cell edge.
The soft frequency reuse scheme was disclosed in CN 1783861, entitled “Method for realizing soft frequency reuse in radio communication system,” and invented by Yang Xuezhi. The power density mask thereof is shown in FIG. 4. FIGS. 5 and 6 show coverage areas of all frequency bands.
In the soft frequency reuse scheme as shown in FIG. 4, the whole bandwidth is divided into three sub-bands, referred to as B1, B2, and B3, respectively. One sub-band is selected as primary sub-band for each cell with a relatively high UPDL, which can cover the entire cell. Other sub-bands are selected as secondary sub-bands with a relatively low UPDL, which can only cover an inner region of the cell. Users at the cell edge can use the primary sub-band of the cell only, while users at the inner region can use both the primary and the secondary sub-bands.
As shown in FIG. 5 or 6, in cell 1, B1 is the primary sub-band, which has a relatively high UPDL and can cover the entire cell; while B2 and B3 are secondary sub-bands with a relatively low UPDL and can cover the inner region of the cell only. Users at the cell edge can merely use the primary sub-band B1, while users at the inner region can use both the primary sub-band B1 and the secondary sub-bands B2 and B3.
In the soft frequency reuse scheme, the primary sub-bands of adjacent cells do not overlap with each other, and users at the edges of the adjacent cells therefore do not interfere with each other. At the cell edge, although there are less bandwidth available compared to reuse 1, the SINR is improved significantly enough to compensate the loss in bandwidth. As a result, the channel capacity at cell edge is largely improved. At the inner region of the cell, the expected signals are relatively strong, while the interferences from neighboring cells are relatively weak. Therefore all bandwidth is used, leading to high spectrum efficiency. Soft frequency reuse, which overcomes the defects of fractional frequency reuse, has now been intensively studied and applied, and become the mainstream technology in the ICIC field.
It can be seen that soft frequency reuse is essentially a network planning technology, providing a framework for resource allocation in each cell, under which an optimized interference pattern is achieved, improving performances of the entire network.
Soft frequency reuse divides communication resource in the frequency dimension. In parallel, communication resource can be divided in the time dimension. Time can be divided into several time slots, each of which is configured with a UPDL. This will lead to the soft time reuse scheme. More generally, communication resource can be divided in the time-frequency plane into many time-frequency blocks, thus forming the soft time-frequency reuse scheme.
The defect of soft frequency reuse lies in that the UPDL of each cell has only two levels, which merely provides a relatively rough restriction on resource allocation in each cell. Thus, there is a still relatively large room for performance improvement.
In two adjacent cells as shown in FIG. 7, the primary sub-band and secondary sub-band of cell 1 are B1 and B2 respectively. At the edge of cell 1, there are two user terminals u11 and u12, respectively using f1 and f2, which both belong to B1, for communication. As u12 is closer to the cell edge than u11, it uses a larger transmit power. The primary sub-band and secondary sub-band of cell 2 are B2 and B1 respectively, and there are two user terminals u21 and u22 at the inner region of cell 2, using f1 and f2 for communication respectively. As u21 is closer to the cell center than u22, it uses a smaller transmit power. Situations are similar in the uplink and downlink. Thus, the interference pattern here is, u11 and u21 interfere with each other, and u12 and u22 interfere with each other.
Such an interference pattern is inferior to that shown in FIG. 8. Since u12 is closer to the cell edge than u11, it is more likely to be interfered by an adjacent cell. Therefore, in a better interference pattern, u12 is placed on the same frequency with u21, while u11 is placed on the same frequency with u22. In such a pattern, user u12 with the poorest transmission condition will be least interfered, thus achieving higher rates; while the largest interference produced by u12 is exerted on user u21 with the best transmission condition. As compared with the interference pattern in FIG. 7, the interference pattern in FIG. 8 improves the lowest rate at the cell edge at the cost of lowering the highest rate in the cell center. This can improve users' experience and reduce complaints, thereby facilitating operations.
However, the existing soft frequency reuse scheme with only two UPDLs cannot provide more accurate restriction to achieve such a superior interference pattern. This is especially true in relatively complex networks. The performance of the system thus still has a relatively large room for improvement.